Filtrate Composition (WJEC GCSE Biology)

Higher Tier Only

• The kidneys filter the blood and the composition of the resulting filtrate changes as it passes through the kidney; these processes occur as follows:

Filtration under pressure

• Small arteries called arterioles lead into and away from the capillary knot within the Bowman’s capsule
• The diameter of the arteriole that leads into the capillary knot is wider than that of the arteriole leaving the capillary knot; this slows the flow of blood as it leaves the knot and results in high blood pressure within the capillaries
• This high pressure forces small molecules out of the capillaries and into the Bowman’s capsule, where they form the filtrate
• The substances forced into the filtrate include:
  • Glucose
  • Water
  • Urea
  • Salts

Filtration under pressure diagram

Blood is filtered at high pressure, forcing small molecules out of the capillary knot and into the filtrate

Selective reabsorption

• Filtrate flows from the Bowman's capsule into the tubule, where useful substances are reabsorbed back into the blood; this is known as selective reabsorption
• Reabsorbed substances include:
  • Glucose
    • This is reabsorbed by active transport
    • Glucose reabsorption occurs in the first part of the tubule
    • In an individual with normal blood glucose levels all of the glucose in the filtrate will be reabsorbed into the blood
  • Salts
    • Some of the salts present in the filtrate are reabsorbed; this depends on the existing water/salt balance in the blood
  • Water
    • Water reabsorption occurs by osmosis
    • The volume of water reabsorbed depends on the water content of the blood
    • Water reabsorption is regulated by the hormone ADH
• Substances that are not reabsorbed from the tubule will flow out of the collecting duct towards the bladder in the urine

Selective reabsorption diagram

Useful substances are selectively reabsorbed back into the blood